In the manufacture of semiconductor devices there are many process steps which involve removal of silicon oxide. A number of known protocols for silicon oxide etch use a gas phase mixture of HF and an enabling chemical.
Under reasonable conditions of temperature below about 100.degree. C. and pressures of about 2 atmospheres or less, gas phase etch of silicon oxide is generally impractical without a second chemical which participates in some way to allow the etch reaction to proceed at a practical rate. A variety of enabling chemicals for facilitating silicon oxide removal with HF are known in the art, including water, alcohols, ketones and some carboxylic acids. Such compounds are liquids at ambient pressure and temperature. Similarly, U.S. Pat. Nos. 5,094,701, 5,221,366, 5,213,621, 5,213,622, and 5,332,444 describe techniques for gas phase removal of metals, metal nitrides or metal oxides which employ HF and a second chemical, such as a beta-diketone or beta-ketoimide, which functions complexing reagent to produce volatile metal-ligand complexes which then can be sublimed from the surface.
As used herein the term "enabling chemical" refers to such second chemicals. The term is not intended to imply any particular mechanism by which the enabling chemical operates in the overall etch scheme. The mechanism of action of the enabling chemical may vary depending on the enabling chemical used. For instance it may be conventionally catalytic, e.g. participating in some transition state but neither being created nor consumed in the overall reaction; autocatalytic, as in the case of water, which lowers the rate of etch initiation but is also a byproduct of the reaction so that reaction rate accelerates as the reaction proceeds; or as a classical coreactant, increasing the etch rate to a practical level because a different, more favored, reaction product is produced when the enabling chemical is present, or complexing with the HF reaction product to produce a volatile reaction product which can be removed in the gas phase.
It has long been known that gas phase HF/water mixtures can be used to etch various silicon oxide films. Early references include J. P. Holmes, et al, "A Vapor Etching Technique for the Photolithography of Silicon Dioxide," Microelectronics and Reliability, 5 pp 337-341 (1966); and K. Breyer, et al, "Etching of SiO.sub.2 in Gaseous HF/H.sub.2 O, " IBM Technical Bulletin, 19(7) (12/1976), both of which used a HF/water azeotrope.
In U.S. Pat. No. 4,749,440 (Blackwood), a process for removing silicon oxide films from silicon wafers using anhydrous HF gas and water vapor carried in a nitrogen stream is disclosed. The gases are mixed just prior to entering a process chamber. The products are gaseous and are removed by the inert nitrogen carrier gas. This process has advantages over previous liquid phase etching procedures, in reducing heavy metals deposits which are often introduced during rinse steps, and in reducing environmental problems. Additionally, the use of anhydrous HF provides improved process control compared to prior gas phase HF/water processes in which HF is supplied as an azeotrope with water. An ambient pressure apparatus for performing this process is currently commercially available from FSI International, Inc., under the trademark Excalibur.RTM..
Various publications describe gas-phase HF/alcohol processes for etching silicon oxide.
U.S. Pat. No. 5,022,961, (Izumi), describes a process for removing a film of a silicon oxide, from a silicon substrate. Two steps are identified:
(a) placing the substrate in a reaction chamber to be isolated in an air-tight manner from the outside air, and PA1 (b) feeding anhydrous hydrogen fluoride and alcohol simultaneously into the reaction chamber. PA1 a) a process chamber isolatable from the atmosphere for receiving and treating the substrate; PA1 b) a gas supply system for providing an etching gas mixture in the process chamber, the gas supply system including PA1 A further aspect of the invention is a gas phase process for etching a substrate in a process chamber using a HF/enabling chemical system, characterized in that both the HF and enabling chemical gases are provided to the chamber at flow rates which are independently controlled relative to each other and to any carrier gas which may be employed.
The reference indicates that the HF/alcohol feeds may be as liquid solutions or gas mixtures. A similar disclosure of an ambient pressure gas phase etch process is contained in A. Izumi, et al, "A New Cleaning Method by Using Anhydrous HF/CH.sub.3 OH Vapor System," J. Ruzyllo et al, ed., Symposium on Cleaning Technology in Semiconductor Device Manufacturing, ECS Proceedings, 92(12), pp 260-266 (1992).
U.S. Pat. No. 5,439,553 (Grant, et al), issued Aug. 8, 1995 from an application filed in the United States on Mar. 30, 1994, describes and claims a low pressure process for removing silicon oxide from a wafer substrate in which an HF/alcohol gas mixture is used at a low pressure to minimize condensation. The same process was earlier published in printed course materials distributed to attendees of a short course entitled "Semi-Conductor Wafer Cleaning Technology" which was held in Austin, Tex. on Feb. 23rd and 24th, 1993, by Werner Kern Associates, East Windsor, N.J. At that short course, one of the inventors of U.S. Pat. No. 5,439,553 also presented a lecture on dry cleaning processes which included a discussion of vapor phase etching of silicon oxide using a HF/methanol process under low pressure conditions where condensation does not occur.
J. Butterbaugh, et al, "Gas Phase Etching of Silicon Oxide with Anhydrous HF and Isopropanol," Proceedings of the Third International Symposium on Cleaning Technology in Semiconductor Device Manufacturing, ECS Proceedings, 94(7) pp 374-383 (1994), describe a low pressure HF/isopropanol etch process for silicon oxide.
Low pressure processes generally are of increasing interest as cluster tools come into use in the semiconductor manufacturing industry. Cluster tools link a series of separate process modules via a central robotic handler, operating at a pressure of about 10 torr or less, allowing substrates such as silicon wafers to undergo multiple sequential processes without exposure to the environment. Such environmental isolation is becoming increasingly important as device features shrink, causing smaller and smaller contaminant regions to become problematic, and ultralarge scale integration increases the investment represented in each defective chip.
In all of the gas phase HF/enabling chemical systems heretofore described, whether operated at ambient pressure or low pressure, the enabling chemical has been provided in a manner which is dependent on a flow of carrier gas, for instance through a bubbler or in a spray vaporizer. None of the prior art systems have provided enabling chemical gas completely independent of a carrier gas flow. Such systems typically also provide an independent carrier gas flow so that a wider variation in HF/enabling chemical/carrier gas ratio is available. However, the adjustment of gas ratios can be quite complicated. For instance in one recently described system, U.S. Pat. No. 5,571,375, an alcohol enabling chemical gas is provided in part as a mixture with nitrogen carrier gas and in part as an azeotrope with HF produced by vaporization of an HF/alcohol solution with a nitrogen bubbler. Make up nitrogen is also provided.
In addition to the complication of such prior art systems, the amount of enabling gas supplied depends in part on the particular conditions of vaporization. Consequently, normal variations in the ambient pressure and temperature in the vaporization device can have significant impact on the proportion of enabling chemical gas in the carrier gas/enabling chemical gas mixture produced.